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    <p>Declaring a <strong>static</strong> variable in Java, means that there will be only one copy, no matter how many objects of the class are created. The variable will be accessible even with no <code>Objects</code> created at all. However, threads may have locally cached values of it.</p> <p>When a variable is <strong>volatile</strong> and not <strong>static</strong>, there will be one variable for each <code>Object</code>. So, on the surface it seems there is no difference from a normal variable but totally different from <strong>static</strong>. However, even with <code>Object</code> fields, a thread may cache a variable value locally.</p> <p>This means that if two threads update a variable of the same Object concurrently, and the variable is not declared volatile, there could be a case in which one of the thread has in cache an old value.</p> <p>Even if you access a <strong>static</strong> value through multiple threads, each thread can have its local cached copy! To avoid this you can declare the variable as <strong>static volatile</strong> and this will force the thread to read each time the global value.</p> <p>However, <strong>volatile</strong> is not a substitute for proper synchronisation!<br> For instance:</p> <pre><code>private static volatile int counter = 0; private void concurrentMethodWrong() { counter = counter + 5; //do something counter = counter - 5; } </code></pre> <p>Executing <code>concurrentMethodWrong</code> concurrently many times may lead to a final value of counter different from zero!<br> To solve the problem, you have to implement a lock:</p> <pre><code>private static final Object counterLock = new Object(); private static volatile int counter = 0; private void concurrentMethodRight() { synchronized (counterLock) { counter = counter + 5; } //do something synchronized (counterLock) { counter = counter - 5; } } </code></pre> <p>Or use the <a href="https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/atomic/AtomicInteger.html" rel="noreferrer"><code>AtomicInteger</code></a> class.</p>
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    1. COSo does it means ALWAYS it's necessary to set volatile?
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    2. COThe volatile modifier guarantees that any thread that reads a field will see the most recently written value, so it's necessary if the variable is shared among multiple thread and you need this feature, it depends on your use case.
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    3. COWhat is the cache when you say "locally cached"? CPU cache, some kind of JVM cache?
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